Conventionally, clamp devices of various types are used in order to fix a work pallet or die to a base member provided on a table, or the like, of a machine tool. For example, the present applicants have developed the following kind of clamp device into practical use.
In this clamp device, a cylindrical holding section is formed in the upper half portion of the clamp main body, and a plurality of steel balls are held in a movable fashion in the radial direction, in this holding section. A piston member forming an output member for outputting a clamping force and a clamp releasing force is inserted movably into this holding section. This piston member is urged downwards by disc springs, for example, and when clamping is released, it is driven upwards by a hydraulic cylinder. A plurality of inclined sections for pressing the plurality of steel balls in an outward direction are provided in the upper end section of the piston member, and a plurality of recess sections into which the plurality of steel balls can retreat are provided respectively in the lower side of each inclined section. On the other hand, a ring-shaped bush is fixed to the work pallet, and a ring-shaped tapered face capable of engaging with the plurality of steel balls is formed on the upper end side portion of the bush.
When the clamping action is released, the piston member is pushed upwards by the hydraulic cylinder, against the elastic urging force of the disc spring. From this state, if the hydraulic pressure in the hydraulic cylinder is discharged, then the piston member will be driven downwards by the elastic urging force of the disc spring, and in association with this, the plurality of steel balls will be pushed outwards in a radial direction, by means of the plurality of inclined sections. The outer portion of each steel ball engages with the ring-shaped tapered face, and a clamping force is transmitted to the bush, whereby the work pallet becomes clamped.
From this clamped state, if hydraulic pressure is supplied to the hydraulic cylinder, then the piston member will be driven upwards by the hydraulic cylinder, and in association with this, the steel balls will retreat respectively towards the inner side, and the inner section of each steel ball will become accommodated inside a recess section, whereby the engagement between the steel balls and the ring-shaped tapered face is released and the fixing of the work pallet is released.
Next, the problems to be solved by the present invention will be described.
In the clamp device described above, a work pallet is clamped by means of a plurality of steel balls engaging with the ring-shaped tapered face of the bush, but since the contact surface area between the respective steel balls and the ring-shaped tapered face is small, the clamping force acts in a localized fashion in the ring-shaped tapered face. Therefore, in particular, when the clamping force is large, the steel balls leave contact marks on ring-shaped tapered face, and as the work pallet is used repeatedly, the clamping force starts to act unevenly on the ring-shaped tapered face, and there is a risk that it will become difficult to maintain a stable clamped state.
Although it is possible to avoid leaving the contact marks due to the steel balls by reducing the force acting locally on the ring-shaped tapered face, in order to increase the contact surface area between the steel balls and the ring-shaped tapered face, the steel balls must be made larger in size, or the number of steel balls must be increased, and hence the clamp device will become larger in size, and disadvantages in manufacturing costs will be incurred.
It is an object of the present invention to provide a clamp device whereby a stable clamped state can be maintained, even if a work pallet or die is used repeatedly, and to provide a clamp device which is compact in size and capable of clamping a work pallet or die by a large clamping force. Further objects of the present invention will become apparent from the description of the advantages of the invention and the description of the embodiments.